1. Field of the Invention
The present invention relates to voltage-controlled oscillators, and more particularly to a voltage-controlled oscillator for use as a temperature-compensated quartz oscillator to be controlled under voltage.
2. Description of the Related Art
Recently, due to the rapid progress of mobile communication apparatuses such as cellular phones, those communication apparatuses are required many functional additions including temperature compensation performance, size reduction, raising use frequency and so on. Accordingly, in such a communication apparatus, there are requirements of temperature compensation performance, size reduction, raising use frequency, etc. for the quartz oscillator for use as a reference of communication frequency.
The temperature-compensation quartz oscillator is a quartz oscillator having a temperature compensation function and reduced in the frequency change due to temperature change, and broadly used as a reference frequency source for a cellular phone or the like. The voltage-controlled oscillator has a variable capacitance element capable of changing the capacitance value based on voltage, as a variable capacitance within an oscillation loop. By controlling the terminal voltage of the variable capacitance element, the oscillator is controlled in frequency by changing the load capacitance value. The temperature-compensation quartz oscillators include those that the temperature characteristic of quartz oscillator (piezoelectric vibrator) is to be cancelled by controlling the variable-capacitance terminal voltage of the voltage-controlled oscillator.
Recently, the effort toward size reduction is proceeded for the temperature-compensation quartz oscillator, in addition to phase noise reduction, startup time reduction, temperature-compensation accuracy increase and so on. The size reduction of quartz oscillator is requisite for realizing the size reduction of quartz oscillator. However, there is a general tendency that the ratio of frequency change to variable capacitance change decreases with reducing the size of quartz oscillator.
Accordingly, there is a need to increase the capacitance change amount relative to a control voltage as to a variable capacitance used as a load capacitance. For example, as shown in JP-A-2003-318417 and JP-A-11-220329, capacitance value change can be taken great for a control voltage change by use of the capacitance caused between the source-drain terminal and the gate terminal of a MOS transistor short-circuited at its source and drain terminals, thus improving the sensitivity increase in frequency change on the quartz oscillator (see FIG. 7).
For example, as shown an example of voltage-controlled oscillator in FIG. 8, there is a proposal having an amplifier having a feedback resistance 1 and inverter 2, a piezoelectric vibrator 3, and first and second MOS transistors 5, 6 connected as variable capacitances to the respective terminals of the piezoelectric vibrator. In the variable capacitance, the first and second MOS transistors 5, 6 are short-circuited at the source-drain terminals. The capacitance, caused between the source-drain terminal and the gate of first and second MOS transistors, is controlled by a voltage source 7 connected to the gate terminal.
In the voltage-controlled oscillator, the capacitance caused between the source-drain terminal and the gate terminal of the MOS transistor is directly connected as a variable capacitance directly to the amplifier and quartz vibrator (piezoelectric vibrator) of an oscillation circuit. Frequency is controlled by changing the capacitance occurring between the source-drain terminal and the gate terminal due to control of the gate voltage of the MOS transistor. In this case, when the gate voltage of MOS transistor becomes a (source-drain terminal voltage + threshold voltage), a channel is formed underneath the gate oxide film, to increase the capacitance between the gate terminal and the channel, i.e. source-drain terminals. (This voltage is taken as a capacitance switchover voltage).
As a first problem of the conventional voltage-controlled oscillator described above, there is a problem that, because the DC bias at the drain terminal is determined on the amplifier side of oscillator circuit, the capacitance switchover voltage cannot be set at a desired value thus making it impossible to control the frequency about a desired gate voltage.
Meanwhile, as a second problem, in the usual CMOS process, the capacitance switchover voltage changes depending upon MOS transistor threshold variation and temperature characteristic. In the prior art, the temperature compensation control signal and external voltage frequency control signal is required to have a characteristic for canceling the MOS transistor threshold variation and temperature characteristic.
Furthermore, as a third problem, there is a problem that, because the capacitance value is great when it is equal to or smaller than the capacitance switchover voltage, frequency variable range is narrow. This is because that the frequency versus capacitance characteristic depicts an exponential curve and hence the frequency variable range relative to capacitance change is narrow in case the capacitance value is great when it is equal to or smaller than the capacitance switchover voltage.
For this reason, in order to facilitate the design of and put into practical use a quartz oscillator using a capacitance caused between the source-drain terminal and the gate terminal of a MOS transistor, there is a need to increase the capacitance occurring between the MOS transistor terminals or increase the capacitance by using an array structure, and further to control the MOS transistor threshold voltage control signal independently of the temperature compensation control signal and external voltage frequency control signal.